The invention relates generally to methods and apparatus for controlling operation of starter/generators such as are often used on engines. More particularly, the invention relates to starter/generator speed sensing control, for example, during an engine start cycle for starter cutoff.
It is well known to use generators as starters for motors and engines. For example, in the aerospace industry, DC generators are commonly used as engine starters on small aircraft engines such as gas turbine engines. After the engine is started, the generator typically is used as an electrical power source for the aircraft.
As a starter/generator for an engine, the generator armature is supplied electrical energy typically from a starter battery. The generator develops substantial torque initially to begin turning the engine.
As the generator speed increases, the generator begins to produce an armature output voltage, commonly referred to as back EMF. Increasing back EMF has the effect of reducing the armature current and hence the starting torque. In some applications, the back EMF may build too quickly before the engine is at idle speed. In such cases, it is generally known to implement a field weakening function in which the field current is forced lower, thereby causing the back EMF to decrease. This in turn increases the armature current to maintain a desired starting output torque from the generator.
Eventually, the starter/generator and engine reach a sufficient speed for full engine operation to begin. At such time, the battery is disconnected from the generator and the generator is connected to a main bus or load, typically through closure of a contactor switching mechanism. Thereafter, the generator functions as a power source driven by the running engine. This switching from a starter to a generator is commonly referred to as starter cutoff. In addition to wanting adequate engine speed at starter cutoff, adequate generator speed is also needed to permit proper voltage regulation of the generator output, as well as to assure that the generator can accommodate the expected load after it is connected to the main bus.
Typically, starter cutoff is accomplished by the use of a separate generator speed sensor, such as a LAPSA 23072-1400. Although the use of such a device permits accurate control of starter cutoff at a selected generator speed, the sensors tend to be expensive, high maintenance items. Also, typical systems are designed so that if the speed sensor is inoperative or otherwise unavailable as a control device, engine start is abnormal or aborted.
The objectives exist, therefore, to provide reliable starter cutoff operation and generator speed sensing functions that can be used as an alternative to or backup for a conventional generator speed sensor.